Athletic shoes normally include a sole for providing traction and cushioning, and an upper for holding the foot of the wearer to the sole. The soles ordinarily have a multi-layer construction comprised of an outsole, a midsole and an insole. The outsole is normally formed of a durable material to resist wearing of the sole during use. The midsole ordinarily forms the middle layer of the sole and is typically composed of a soft foam material to cushion the impact forces and pressure experienced by the foot during athletic activities. The material used for the foam midsole typically has a Shore A hardness of approximately 55-60. The foam midsole may be formed with or without the inclusion of other cushioning elements, such as a resilient inflated bladder. An insole layer is usually a thin padded member provided overtop of the midsole to enhance the comfort afforded to the wearer.
Most shoes, including athletic shoes, are designed so that the medial side and the lateral side of the shoe are symmetric. This includes shoes designed for the sport of tennis which are routinely symmetrically designed. However, such designs do not take into account the demands and requirements of the sport as they relate to the performance and safety of the tennis player. For example, motions prevalent in tennis footwork create instability leading to fatigue, injury and inefficiency of footwork action. Accordingly, an article of footwear for use in the sport of tennis that maximizes performance and minimizes injury was thus needed. Additionally, U.S. Pat. No. 4,694,591 discloses a split midsole design. However, such a design would be unsuitable for the sport of tennis due to the typical movements involved.
In most prior art shoes, the outsole includes flex grooves that are either (a) straight across the sole, i.e., they are transverse to the longitudinal axis of the shoe and the foot of the wearer or (b) angled by a curve created by the metatarsal heads, see, e.g., U.S. Pat. No. 4,559,724. However, the grooves in these directions may be undesirable in some sports, for example tennis, where these groove are designed to flex in a direction generally opposite of the direction of many movements.